libucontext: use hard/soft float

libucontext: use hard/soft float

When building for qemuarm-secureboot in meta-arn with musl and clang,
the following compile error is seen:
- | ../sources/libucontext-1.3.2/arch/arm/swapcontext.S:23:11: error: unknown token in expression
- |  ldr r4, =#0x56465001
- |           ^

This is happening because 1.3 added ifdefs for assembly code for both
hard and soft float, and bcause neither is being defined, it is taking
this path with the issue.

Since we can tell if soft or hard float is being used via the TARGET_FPU
variable, use that and set the relevant makefile flag.

Signed-off-by: Jon Mason <jon.mason@arm.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>