arm64: entry: Simplify tramp_alias macro and tramp_exit routine

arm64: entry: Simplify tramp_alias macro and tramp_exit routine

The tramp_alias macro constructs the virtual alias of a symbol in the
trampoline text mapping, based on its kernel text address, and does so
in a way that is more convoluted than necessary. So let's simplify that.

Also, now that the address of the vector table is kept in a per-CPU
variable, there is no need to defer the load and the assignment of
VBAR_EL1 to tramp_exit(). This means we can use a PC-relative reference
to the per-CPU variable instead of storing its absolute address in a
global variable in the trampoline rodata.

And given that tramp_alias no longer needs a temp register, this means
we can restore X30 earlier as well, and only leave X29 for tramp_exit()
to restore.

While at it, give some related symbols static linkage, considering that
they are only referenced from the object file that defines them.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230418143604.1176437-3-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>