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

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.