ArmPlatformPkg/ArmPlatformStackLib: use callee preserved registers
The entry code in ArmPlatformStackSet () is a 1:1 transliteration of the
ARM version, which uses the callee preserved registers r3 - r7 (*) to
preserve the function arguments and the link register across a call to
ArmPlatformIsPrimaryCore ().
However, x3 - x7 are not callee preserved on AARCH64, and so we should use
registers >= x19 instead. While we're at it, drop an unnecessary preserve
of the link register, and simplify/deobfuscate the calculation of the
secondary stack position.
(*) Note that r3 is not actually a callee saved register even on ARM.