Remove architecture specific shmat.S files.

[ali_mashtizadeh.com]

Summary

These files are identical to the generated system calls.
In the case of MIPS, the file was already removed from the build.

Test Plan

make universe

[ali_mashtizadeh.com]

--- universe_kernconf_arm_IMX6 ---
--- universe_kernconf_arm_LINT ---
--- universe_kernconf_arm_PANDABOARD ---
--- universe_kernconf_arm_RPI-B ---
--- universe_kernconf_arm_SAM9G20EK ---
--- universe_kernconf_arm_SOCFPGA ---
--- universe_kernconf_arm_TEGRA124 ---
--- universe_arm64_aarch64 ---
>> arm64.aarch64 buildworld completed on Thu Apr  5 06:33:36 EDT 2018
--- universe_arm64_kernels ---
--- universe_kernconf_arm64_GENERIC ---
--- universe_kernconf_arm64_GENERIC-NODEBUG ---
--- universe_kernconf_arm64_GENERIC-UP ---
--- universe_arm_kernels ---
--- universe_kernconf_arm_TS7800 ---
--- universe_kernconf_arm_VERSATILEPB ---
--- universe_kernconf_arm_VIRT ---
--- universe_kernconf_arm_VYBRID ---
--- universe_kernconf_arm_ZEDBOARD ---
--- universe_arm64_done ---
>> arm64 completed on Thu Apr  5 07:16:19 EDT 2018
--- universe_arm_done ---
>> arm completed on Thu Apr  5 07:16:25 EDT 2018
--- universe_amd64_amd64 ---
>> amd64.amd64 buildworld completed on Thu Apr  5 07:22:05 EDT 2018
--- universe_amd64_kernels ---
--- universe_kernconf_amd64_GENERIC ---
--- universe_kernconf_amd64_GENERIC-MMCCAM ---
--- universe_kernconf_amd64_GENERIC-NODEBUG ---
--- universe_kernconf_amd64_LINT ---
--- universe_kernconf_amd64_LINT-NOINET ---
--- universe_kernconf_amd64_LINT-NOINET6 ---
--- universe_kernconf_amd64_LINT-NOIP ---
--- universe_kernconf_amd64_MINIMAL ---
--- universe_i386_i386 ---
>> i386.i386 buildworld completed on Thu Apr  5 07:55:28 EDT 2018
--- universe_i386_kernels ---
--- universe_kernconf_i386_GENERIC ---
--- universe_kernconf_i386_GENERIC-NODEBUG ---
--- universe_kernconf_i386_LINT ---
--- universe_kernconf_i386_LINT-NOINET ---
--- universe_kernconf_i386_LINT-NOINET6 ---
--- universe_kernconf_i386_LINT-NOIP ---
--- universe_kernconf_i386_PAE ---
--- universe_amd64_done ---
>> amd64 completed on Thu Apr  5 08:24:16 EDT 2018
--- universe_i386_done ---
>> i386 completed on Thu Apr  5 08:40:23 EDT 2018
--- universe_epilogue ---
--------------------------------------------------------------
>>> make universe completed on Thu Apr  5 08:40:23 EDT 2018
                      (started Wed Apr  4 23:01:07 EDT 2018)
--------------------------------------------------------------

[kib]

It seems it was brought in with r129202 "arm libc bits from NetBSD", then copied to arm64 and from there to riscv.

[brooks]

LGTM. I'll take care of committing this.