[binutils-gdb] gdb/riscv: Fix failures on rv64 in gdb.arch/riscv-reg-aliases.exp test

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT 92dcebf3fa5ccc1cae8fcb93eaf08ffea52957e2 ***

Author: Andrew Burgess <andrew.burgess@embecosm.com>
Branch: master
Commit: 92dcebf3fa5ccc1cae8fcb93eaf08ffea52957e2

gdb/riscv: Fix failures on rv64 in gdb.arch/riscv-reg-aliases.exp test

The gdb.arch/riscv-reg-aliases.exp test didn't take into account that
on RV64 (and RV128) the floating point registers are represented as a
union.  This patch updates the test to handle this.

Tested against RV32 and RV64.

gdb/testsuite/ChangeLog:

	* gdb.arch/riscv-reg-aliases.exp: Rewrite to take account of float
	registers being unions.