[kernel][make] move WITH_NO_FP define into KERNEL_DEFINES instead of command line

No functional change, just an itch to scratch.

Change-Id: I20abf227752567d04c08accba03295f794c55090

[build] Use CLANG_TOOLCHAIN_PREFIX for Clang

This allows mixing GCC and Clang toolchains in a single build, which
will be useful for building host tools or bootloader.

This also allows ld and gold from GCC toolchain even when building
with Clang which is necessary until lld is capable of linking Magenta.

Change-Id: I756d61de82adbd88f1b7cc1ded1ed4bff5e016c8

[build] Use boolean flags to enable Clang

This is to match other flags used by the build.

Change-Id: Idd1c1714aff3d03dbc58b92a2cbe0ee0a75d8089

[magenta] Initial commit