Documentation: raise minimum supported version of GCC to 5.1

commit fad7cd3 ("nbd: add the check to prevent overflow in
__nbd_ioctl()") raised an issue from the fallback helpers added in
commit f090782 ("compiler.h: enable builtin overflow checkers and
add fallback code")

Specifically, the helpers for checking whether the results of a
multiplication overflowed (__unsigned_mul_overflow,
__signed_add_overflow) use the division operator when
!COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW.  This is problematic for 64b
operands on 32b hosts.

Also, because the macro is type agnostic, it is very difficult to write
a similarly type generic macro that dispatches to one of:
 * div64_s64
 * div64_u64
 * div_s64
 * div_u64

Raising the minimum supported versions allows us to remove all of the
fallback helpers for !COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW, instead
dispatching the compiler builtins.

arm64 has already raised the minimum supported GCC version to 5.1, do
this for all targets now.  See the link below for the previous
discussion.

Link: https://lore.kernel.org/all/[email protected]/
Link: https://lore.kernel.org/lkml/CAK7LNASs6dvU6D3jL2GG3jW58fXfaj6VNOe55NJnTB8UPuk2pA@mail.gmail.com/
Link: ClangBuiltLinux#1438
Reported-by: Stephen Rothwell <[email protected]>
Reported-by: Nathan Chancellor <[email protected]>
Suggested-by: Rasmus Villemoes <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Documentation/process/changes.rst

@@ -29,7 +29,7 @@ you probably needn't concern yourself with pcmciautils.
 ====================== ===============  ========================================
         Program        Minimal version       Command to check the version
 ====================== ===============  ========================================
-GNU C                  4.9              gcc --version
+GNU C                  5.1              gcc --version
 Clang/LLVM (optional)  10.0.1           clang --version
 GNU make               3.81             make --version
 binutils               2.23             ld -v

scripts/min-tool-version.sh

@@ -17,13 +17,7 @@ binutils)
 	echo 2.23.0
 	;;
 gcc)
-	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63293
-	# https://lore.kernel.org/r/[email protected]
-	if [ "$SRCARCH" = arm64 ]; then
-		echo 5.1.0
-	else
-		echo 4.9.0
-	fi
+	echo 5.1.0
 	;;
 icc)
 	# temporary