arm64: uapi: expose our struct ucontext to the uapi headers

arm64 defines its own ucontext structure which is incompatible with the
struct defined (and exposed to userspace by) the asm-generic headers.

glibc carries its own struct definition that is compatible with the
arm64 definition, but we should expose our format in the uapi headers in
case other libraries want to make use of the ucontext pushed as part of
an arm64 sigframe.

This patch moves the arm64 asm/ucontext.h to the uapi headers, along
with the necessary #include of linux/types.h.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Marcus Shawcroft <marcus.shawcroft@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>