[PATCH] Native spinlock support on RISC-V

Hello,

The attached patch adds native spinlock support to PostgreSQL on RISC-V
systems. As suspected by Richard W.M. Jones of Red Hat back in 2016, the
__sync_lock_test_and_set() approach applied on arm and arm64 works here
as well.

Tested against PostgreSQL 13.3 on a physical rv64gc system (BeagleV
Starlight beta board) - builds and installs fine, all tests pass. From
what I can see in gcc documentation this should in theory work on rv32
(and possibly rv128) as well, therefore the patch as it stands covers
all RISC-V systems (i.e. doesn't check the value of __risc_xlen) - but I
haven't confirmed this experimentally.

--
MS