fhandler_proc/cpuinfo: support fast short REP MOVSB

fhandler_proc/cpuinfo: support fast short REP MOVSB

Added in Linux 5.6:
Check FSRM and use REP MOVSB for short copies on systems that have it.

>From the Intel Optimization Reference Manual:

3.7.6.1 Fast Short REP MOVSB
Beginning with processors based on Ice Lake Client microarchitecture,
REP MOVSB performance is enhanced with string lengths up to 128 bytes.
Support for fast-short REP MOVSB is indicated by the CPUID feature flag:
CPUID [EAX=7H, ECX=0H).EDX.FAST_SHORT_REP_MOVSB[bit 4] = 1.
There is no change in the REP STOS performance.