c++: Fix mangling of _Float16 template args [PR121801]

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>

gcc/testsuite/ChangeLog:

	PR c++/121801
	* g++.dg/abi/pr121801.C: New test.

gcc/cp/ChangeLog:

	PR c++/121801
	* mangle.cc (write_real_cst): Handle 16-bit real and assert
	that reals have 16 bits or a multiple of 32 bits.

(cherry picked from commit 19d1c7c28f4fd0557dd868a7a4041b00ceada890)
2 files changed