ArmPkg/DebugAgentSymbolsBaseLib: remove exception handling

ArmPkg/DebugAgentSymbolsBaseLib: remove exception handling

DebugAgentSymbolsBaseLib is an optional library that is in charge
of extracting debug headers from SEC and PEI_CORE images in memory
so the filename and the offset in memory can be reported via the
UART, allowing a developer to load debugging symbols into his
debugger.

Interestingly enough, DebugAgentSymbolsBaseLib is also in charge of
exception handling before this duty is taken over by either the PEI
core, or the CPU DXE driver when running under PrePi.

Since exceptions are not actually handled at all on AArch64, and simply
routed to the DefaultExceptionHandlerLib (for which a special version
has been created to be usable this early), let's get rid of this
dubious functionality altogether.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>