KVM: selftests: Reuse GPR switching logic for nVMX

Reuse the GPR switching logic for nVMX by defining VMX_SWITCH_GPRS_ASM,
which is essentially the same as SVM_SWITCH_GPRS_ASM but also switches
RAX and doesn't switch RFLAGS, replacing the push/pop of a subset of the
registers.

The long clobber list of registers is no longer needed as registers are
saved and restored appropriately (and not clobbered by L2).

Define VMX_SWITCH_GPRS_ASM before including evmcs.h, such that it can be
used by evmcs_vmlaunch() and evmcs_vmresume().

This replaces the apparently thread-safe push/pop sequence with the
global GPR switching logic used by SVM, which isn't thread-safe at all.

However this is still an improvement because:
- The VMX logic is half-baked and prompts the UCALL clobber hack as it
  doesn't properly save/restore everything. Reusing the GPR switching
  logic used by SVM allows for dropping that hack.

- Hitting a problem due to half-baked GPR save/restore logic is arguably
  more likely than thread-safety. Evidently, adding more involved stress
  tests fails on VMX with the existing push/pop sequence. OTOH, there
  are no known failures on SVM due to lack of thread-safety fo
  save/restore. Only one test currently uses more than one vCPU with
  nested (the memstress test).

The logical next step is to move the guest_regs to be per-vCPU,
making it thread-safe for both VMX and SVM in a proper way.

Assisted-by: Gemini:gemini-3.1-pro
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260728174232.2423257-6-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

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