ARM: 9198/1: spectre-bhb: simplify BPIALL vector macro

The BPIALL mitigation for Spectre-BHB adds a single instruction to the
handler sequence that doesn't clobber any registers. Given that these
sequences are 10 instructions long, they don't fit neatly into a
cacheline anyway, so we can simply move that single instruction to the
start of the unmitigated one, and rearrange the symbol names accordingly.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>