Add (unratified) Zfa extension

Zfa adds 'missing' floating-point functionality:
 - fli.[hsdq]: floating-point load-immediate
 - fminm/fmaxm.[hsdq]: IEEE 754-2019 minimum and maximum
 - fround(nx)?.[hsdq]: IEEE 754-2019 roundToIntegral(Exact)?
 - fcvtmod.w.d: ECMAScript modular float->integer conversion
 - move instructions for XLEN < FLEN
 - comparisons that support quiet NaN inputs

This adds a surprisingly large number of files, as the instructions
added with Zfa depend on context (e.g., .h-variants become available
only if Zfh is present).

unratified/rv32_d_zfa

@@ -0,0 +1,2 @@
+fmvh.x.d     rd rs1 24..20=1 31..27=0x1C 14..12=0 26..25=1 6..2=0x14 1..0=3
+fmvp.d.x     rd rs1 24..20=1 31..27=0x16 14..12=0 26..25=1 6..2=0x14 1..0=3

unratified/rv64_q_zfa

@@ -0,0 +1,2 @@
+fmvh.x.q     rd rs1 24..20=1 31..27=0x1C 14..12=0 26..25=3 6..2=0x14 1..0=3
+fmvp.q.x     rd rs1 24..20=1 31..27=0x16 14..12=0 26..25=3 6..2=0x14 1..0=3

unratified/rv_d_zfa

@@ -0,0 +1,8 @@
+fli.d        rd rs1 24..20=1 31..27=0x1E 14..12=0 26..25=1 6..2=0x14 1..0=3
+fminm.d      rd rs1 rs2      31..27=0x05 14..12=2 26..25=1 6..2=0x14 1..0=3
+fmaxm.d      rd rs1 rs2      31..27=0x05 14..12=3 26..25=1 6..2=0x14 1..0=3
+fround.d     rd rs1 24..20=4 31..27=0x08 rm       26..25=1 6..2=0x14 1..0=3
+froundnx.d   rd rs1 24..20=5 31..27=0x08 rm       26..25=1 6..2=0x14 1..0=3
+fcvtmod.w.d  rd rs1 24..20=8 31..27=0x18 14..12=1 26..25=1 6..2=0x14 1..0=3
+fleq.d       rd rs1 rs2      31..27=0x14 14..12=4 26..25=1 6..2=0x14 1..0=3
+fltq.d       rd rs1 rs2      31..27=0x14 14..12=5 26..25=1 6..2=0x14 1..0=3

unratified/rv_f_zfa

@@ -0,0 +1,7 @@
+fli.s        rd rs1 24..20=1 31..27=0x1E 14..12=0 26..25=0 6..2=0x14 1..0=3
+fminm.s      rd rs1 rs2      31..27=0x05 14..12=2 26..25=0 6..2=0x14 1..0=3
+fmaxm.s      rd rs1 rs2      31..27=0x05 14..12=3 26..25=0 6..2=0x14 1..0=3
+fround.s     rd rs1 24..20=4 31..27=0x08 rm       26..25=0 6..2=0x14 1..0=3
+froundnx.s   rd rs1 24..20=5 31..27=0x08 rm       26..25=0 6..2=0x14 1..0=3
+fleq.s       rd rs1 rs2      31..27=0x14 14..12=4 26..25=0 6..2=0x14 1..0=3
+fltq.s       rd rs1 rs2      31..27=0x14 14..12=5 26..25=0 6..2=0x14 1..0=3

unratified/rv_q_zfa

@@ -0,0 +1,7 @@
+fli.q        rd rs1 24..20=1 31..27=0x1E 14..12=0 26..25=3 6..2=0x14 1..0=3
+fminm.q      rd rs1 rs2      31..27=0x05 14..12=2 26..25=3 6..2=0x14 1..0=3
+fmaxm.q      rd rs1 rs2      31..27=0x05 14..12=3 26..25=3 6..2=0x14 1..0=3
+fround.q     rd rs1 24..20=4 31..27=0x08 rm       26..25=3 6..2=0x14 1..0=3
+froundnx.q   rd rs1 24..20=5 31..27=0x08 rm       26..25=3 6..2=0x14 1..0=3
+fleq.q       rd rs1 rs2      31..27=0x14 14..12=4 26..25=3 6..2=0x14 1..0=3
+fltq.q       rd rs1 rs2      31..27=0x14 14..12=5 26..25=3 6..2=0x14 1..0=3

unratified/rv_zfh_zfa

@@ -0,0 +1,7 @@
+fli.h        rd rs1 24..20=1 31..27=0x1E 14..12=0 26..25=2 6..2=0x14 1..0=3
+fminm.h      rd rs1 rs2      31..27=0x05 14..12=2 26..25=2 6..2=0x14 1..0=3
+fmaxm.h      rd rs1 rs2      31..27=0x05 14..12=3 26..25=2 6..2=0x14 1..0=3
+fround.h     rd rs1 24..20=4 31..27=0x08 rm       26..25=2 6..2=0x14 1..0=3
+froundnx.h   rd rs1 24..20=5 31..27=0x08 rm       26..25=2 6..2=0x14 1..0=3
+fleq.h       rd rs1 rs2      31..27=0x14 14..12=4 26..25=2 6..2=0x14 1..0=3
+fltq.h       rd rs1 rs2      31..27=0x14 14..12=5 26..25=2 6..2=0x14 1..0=3