arm: reformat -march option section in gas documentation

arm: reformat -march option section in gas documentation

Hi,

This patch contains a reformatting of -march option section in gas documentation.

For instance (see https://sourceware.org/binutils/docs-2.41/as.html#ARM-Options),
before all the options were on one line:
   For armv8-a:
   +crc: Enables CRC32 Extension. +simd: Enables VFP and NEON for Armv8-A. +crypto: Enables
   Cryptography Extensions for Armv8-A, implies +simd. +sb: Enables Speculation Barrier
   Instruction for Armv8-A. +predres: Enables Execution and Data Prediction Restriction
   Instruction for Armv8-A. +nofp: Disables all FPU, NEON and Cryptography Extensions.
   +nocrypto: Disables Cryptography Extensions.

Now, the readability is improved thanks to the itemization of the options:
   For armv8-a:
    +crc: Enables CRC32 Extension.
    +simd: Enables VFP and NEON for Armv8-A.
    +crypto: Enables Cryptography Extensions for Armv8-A, implies +simd.
    +sb: Enables Speculation Barrier Instruction for Armv8-A.
    +predres: Enables Execution and Data Prediction Restriction Instruction for Armv8-A.
    +nofp: Disables all FPU, NEON and Cryptography Extensions.
    +nocrypto: Disables Cryptography Extensions.

Ok for binutils-master? I don't have commit access so I need someone to commit on my behalf.

Regards,
Matthieu.