Fix emitting CodeView information for locals

[AntonLapounov]

• Emit correct CodeView register numbers for ARM64. Support x86 and ARM as well.
• Stop emitting uninitialized Hdr's fields to the PDB file.

Note that ordering of special ARM64 registers is different in cordebuginfo.h and cvconst.h from DIA SDK:

Header file	ARM64 register order
cordebuginfo.h	FP, LR, SP, PC
cvconst.h	FP, LR, SP, ZR, PC

I am not sure we can insert the ZR register into the RegNum enumeration in cordebuginfo.h without creating a potential mess between different .NET versions. For instance, the debugger uses the hard-coded value of REGNUM_AMBIENT_SP, which would change after the insertion:

runtimelab/src/coreclr/debug/inc/dbgipcevents.h

Line 1878 in 3d3b11a

#define DBG_TARGET_REGNUM_AMBIENT_SP 34

For that reason I handle ARM64's PC register separately. Of course, PC is very unlikely to be used for locals, so we might stop at SP instead.

[jkotas]

Thank you!