Remove ROM routines from libc.a, save progmem (esp8266#6432)

Dropped routines form libc.a which are present and usable in ROM:
`ar dv libc.a lib_a-strcmp.o lib_a-strlen.o lib_a-strncmp.o lib_a-strstr.o lib_a-memcmp.o lib_a-memcpy.o lib_a-memmove.o lib_a-memset.o lib_a-bzero.o`

Left strcpy and strncpy in libc.a because they silently support PROGMEM
accesses as required by GCC optimizations.

Saves ~628 bytes in AdvancedWebServer example, from IROM (not IRAM).

Also allows mem* and str* routines to be safely called from inside an
ISR.  Prior to this PR, these routines were stored in flash and not
IRAM, so they were technically illegal to call while in an ISR.

Fixes esp8266#6430