Allow default construction of BackingMemory on Android
Allow default construction of BackingMemory on Android

CL:32489 caused a compilation error on Android since it
implicitly deletes BackingMemory's default constructor.

Change-Id: I48d415aed8ad0166d8d2bd61d06d9ad5ba8a565c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32629
Kokoro-Presubmit: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Chris Forbes <c3279c50043e18e9442c1ae071cafa0e7a4ca967@google.com>
Tested-by: Hernan Liatis <aae823fecbd46cfee57ca49420b0f7c28a5fd12e@google.com>

bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader