BaseTools/GenFw: Correct offset when relocating an ADR

In the R_AARCH64_ADR_GOT_PAGE case on AARCH64, we may encounter an ADR
instead of an ADRP when the toolchain is working around Cortex-A53
erratum #843419.  If that's the case, be sure to calculate the offset
appropriately.

This resolves an issue experienced when building a StandaloneMm image
with stack protection enabled on GCC compiled with
"--enable-fix-cortex-a53-843419".  This scenario sometimes generates an
ADR with a R_AARCH64_ADR_GOT_PAGE relocation.

In this scenario, the following code is being generated by the
toolchain:

    # Load to set the stack canary
    2ffc:	10028020 	adr	x0, 8000 <mErrorString+0x1bc>
    3008:	f940d400 	ldr	x0, [x0, #424]

    # Load to check the stack canary
    30cc:	b0000020 	adrp	x0, 8000 <mErrorString+0x1bc>
    30d0:	f940d400 	ldr	x0, [x0, #424]

GenFw rewrote that to:

    # Load to set the stack canary
    2ffc:	10000480 	adr	x0, 0x308c
    3008:	912ec000 	add	x0, x0, #0xbb0

    # Load to check the stack canary
    30cc:	f0000460 	adrp	x0, 0x92000
    30d0:	912ec000 	add	x0, x0, #0xbb0

Note that we're now setting the stack canary from the wrong address,
resulting in an erroneous stack fault.

After this fix, the offset will be calculated correctly for an ADR and
the stack canary is set correctly.

Signed-off-by: Jake Garver <jake@nvidia.com>

BaseTools/Source/C/GenFw/Elf64Convert.c

@@ -1562,7 +1562,27 @@ WriteSections64 (
             // subsequent LDR instruction (covered by a R_AARCH64_LD64_GOT_LO12_NC
             // relocation) into an ADD instruction - this is handled above.
             //
-            Offset = (Sym->st_value - (Rel->r_offset & ~0xfff)) >> 12;
+            // In order to handle Cortex-A53 erratum #843419, the GCC toolchain
+            // may convert an ADRP instruction at the end of a page (0xffc
+            // offset) into an ADR instruction. If so, be sure to calculate the
+            // offset for an ADR instead of ADRP.
+            //
+            if ((*(UINT32 *)Targ & BIT31) == 0) {
+              //
+              // Calculate the offset for an ADR.
+              //
+              Offset = (Sym->st_value & ~0xfff) - Rel->r_offset;
+              if (Offset < -0x100000 || Offset > 0xfffff) {
+                Error (NULL, 0, 3000, "Invalid", "WriteSections64(): %s  due to its size (> 1 MB), unable to relocate ADR.",
+                  mInImageName);
+                break;
+              }
+            } else {
+              //
+              // Calculate the offset for an ADRP.
+              //
+              Offset = (Sym->st_value - (Rel->r_offset & ~0xfff)) >> 12;
+            }
 
             *(UINT32 *)Targ &= 0x9000001f;
             *(UINT32 *)Targ |= ((Offset & 0x1ffffc) << (5 - 2)) | ((Offset & 0x3) << 29);