alpha: Do non-default symbols in pt-vfork.S
aarch64: Fix DO_CALL block comment
aarch64: Remove DOARGS/UNDOARGS macros

While they do something for AArch32, they're useless for AArch64.
aarch64: Tidy syscall error check

Move the error branch from the PSEUDO_RET macro to the PSEUDO macro.
This is in line with other architectures, and will enable further improvments.
arm,aarch64: Remove SINGLE_THREAD_P_PIC

This macro was removed by

2005-11-16  Daniel Jacobowitz  <dan@codesourcery.com>

but not applied to the (still separate) eabi port so necro'd
when the eabi port superceded the old abi.  It was thence
copied into the new AArch64 port.